It seems the average non-native speaker of English only has a vocabulary size of about 4,500 words. And over half of these learners will have a vocabulary size of greater than 7,826 words. This is a somewhat depressing picture for language learners considering the worst of native speaker adults still will have a minimum size of 20,000 words with the high end at 35,000 words.
So what is the best way for non-native speakers to increase their vocabulary size? According to these statistics from testyourvocab.com the biggest factor is out-of-class activities. The more you do outside of the classroom the broader your vocabulary. Otherwise three years living abroad will do the trick. Even then the non-native speaker will only have 10,000 words (that is equal to the vocabulary size of an 8 year old native speaker). To reach 18,000 words over 10 years of living abroad is necessary.